The present invention relates to novel toner agents in general and, in particular, to metal-containing complexes, e.g. with both an aromatic dicarboxylic acid and an aromatic hydroxy-carboxylic acid, usable in toners for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing, etc.
Conventional processes for converting latent electrostatic images to visible images are generally divided into two groups: liquid developing processes which use a developer comprising an electrically insulating liquid and a finely divided toner dispersed therein, and dry developing processes, such as the cascade process, fur brush process, magnetic brush process and powder-cloud process, in which a finely divided toner prepared by dispersing a coloring agent in a natural or synthetic resin is used singly or admixed with a solid carrier. The toners useful for such processes are charged positively or negatively in accordance with the polarity of the latent electrostatic image to be developed.
The toner can be made to retain electric changes by utilizing the triboelectric properties of the resin component of the toner, but since the toner is not highly chargeable by this method, the toner image obtained by development is prone to fogging and in turn to being obscure. To give the desired triboelectric properties to the toner, dyes and pigments for affording enhanced chargeability, and furthermore charge control agents (triboelectrification control agents), are added to the toner. Presently used in the art for this purpose are oil-soluble nigrosine dyes for positively charging toners as disclosed in Published Examined Japanese patent application No. 2427/1966, and metal-containing complex dyes for negatively charging toners as disclosed in Published Examined Japanese patent application No. 26478/1970.
However, such dyes and pigments serving as charge control agents are complex in structure and have low stability. For example, they decompose or deteriorate when subjected to mechanical friction and impact, variations in temperature and humidity conditions, electric impact, irradiation with light, etc. or they decompose at about 150.degree. C. when kneaded with a melt. Thus, these known dyes and pigments, because of their low stability, are liable to lose their charge controlling properties. Furthermore, many of these known dyes and pigments have low compatibility with the resin component of the toner, and are therefore difficult to disperse uniformly in the toner and tend to permit uneven charging, with the result that fog occurs in the developed toner image to obscure the image. Moreover, even when the developer exhibits satisfactory developing characteristics in the initial stage of use, the dye or pigment has the drawback of becoming decomposed or of deteriorating with the increase in the number of copying cycles, ultimately rendering the toner no longer serviceable for its contemplated use.
Additionally, one of the substantial drawbacks of conventional dyes or pigments for controlling charges or imparting chargeability is that they are themselves colored materials. This is in conflict with the basic requirement that charge control agents or chargeability imparting agents which are colorless or have a pale color that can be regarded as substantially colorless must be used for toners having a specific color. Recently published Unexamined Japanese patent application No. 127726/1978 discloses a metal complex of salicylic acid or alkylsalicylic acid as a material fulfilling the aforesaid basic requirement, but it is impossible to knead fully this material with a melt of the resin component of the toner at a high temperature sufficient to dissolve or disperse the material uniformly therein since this metal complex material has an unfortunately low thermal stability.